1. Field of the Invention
The present invention relates to an optical disk apparatus for performing at least one of recording and reproducing of signals by using near-field light, and to a control method therefor.
2. Description of the Related Art
In recent years, to enhance a recording density at which signals are recorded on an optical disk by using laser light, there has been proposed an optical disk apparatus that records or reproduces signals by using near-field light. In the optical disk on which signals are recorded by using near-field light, it is necessary to control the length of the gap between an optical disk and an end surface of an SIL (Solid Immersion Lens) installed in a head including an objective lens section in such a way as to be reduced to a distance at which the generation of near-filed light is enabled. Generally, this distance is half the wavelength of input laser light. For example, in the case of using 400 nm wavelength blue-violet laser beams, this distance is about 200 nm.
Thus, an overshoot, which is not problematical for far-field optical systems, such as a DVD (Digital Versatile Disk), and which is caused if the length of the gap is equal to or less than 1 μm at the starting of the control of the gap, is problematical for optical recording/reproducing apparatuses using near-field light. In other words, even if an overshoot occurs if the gap is equal to or less than 1 μm at the starting of control of the gap, the SIL collides with the disk. This causes damage to both the SIL and the disk.
To solve this problem, a method of controlling the gap based on a quantity of return laser light, which is reflected by the disk, is used. For instance, in a case where 400 nm wavelength laser light is used, generally, the length of the gap, at which a near-field condition occurs, is equal to or less than half the wavelength thereof. Therefore, if the length of the gap is more than 200 nm, that is, in a far-field condition, all light from a laser light source, which is incident upon the end surface of the SIL at an angle at which total reflection thereof occurs, is reflected on the end surface of the SIL so that a quantity of return light is constant. However, if the length of the gap is equal or less than 200 nm, that is, in the near-field condition, a part of light being incident upon the end surface of the SIL at the angle, at which total reflection occurs, penetrates through the end surface of the SIL. Thus, a total-reflection return-light quantity decreases. Further, if the length of the gap between the SIL and the disk is zero, that is, when the SIL touches the disk, all the light being incident upon the end surface of the SIL at the angle, at which total reflection occurs, penetrates through the end surface of the SIL, so that the total-reflection return-light quantity is zero. According to this technique, this total-reflection return-light quantity is detected by a photodetector. Then, a gap servo operation is performed on the SIL by feeding back the detected total-reflection return-light quantity to an actuator (for example, a 2-axis device for performing a focusing servo operation and a tracking servo operation) for the SIL (see, for instance, Japanese Patent Application Publication 2002-76358, Paragraph 0026 and FIG. 3).
Furthermore, there is another method used. According to this method, a threshold (a gap servo starting threshold value) for identifying a near-field condition is set. Then, the SIL is made to slowly approach the disk. A gap servo operation would not start until the total-reflection return-light quantity becomes smaller than a gap servo starting threshold value. In other words, the gap servo operation would not start until the distance therebetween becomes equal to a near-field distance.